In a steering column which constitutes a vehicle steering apparatus, at an axial central portion of a steering column which is built into a height position adjustment device for a steering wheel referred to as a tilt type steering device, or which is built into a fore and aft position adjustment device for a steering wheel referred to as a telescopic steering device, it is necessary to fix a bracket referred to as a column bracket thereto. Heretofore, typically such a column bracket which is formed separate to the steering column, is welded and secured to the steering column later on. On the other hand, in Patent Document 1 identified below, a construction is disclosed where, as shown in FIG. 15 and FIG. 16, an axial central portion of a hollow tube made of metal constituting a steering column 1 is expanded in the radial outward direction, and this expanded portion is made a column bracket 2. By adopting such a construction, the number of parts is reduced, and a light weight low cost vehicle steering apparatus can be realized.
In order to make a steering column 1 with such a column bracket 2 provided integrally, a hydroforming process is used to apply hydraulic pressure (for example water pressure) to the inner peripheral surface of a metal tube 3 (made of steel or aluminum alloy) which constitutes the steering column 1, and expand (plastically deform) one part of the metal pipe 3, as shown in FIGS. 15 and 16, in the radial outward direction. In expanding the axial central portion of the metal tube 3 by the aforementioned hydroforming process, for example as shown in FIG. 19 described later, a hollow member 11 (metal pipe) being the primary member is set inside a separable die 6 which has an inner surface shape which matches with an outer surface shape of the aforementioned metal tube 3 for which the diameter is to be expanded. Then the opposite ends of the hollow member 11 are closed by axial pushing tools 19a and 19b, and high pressure liquid of for example 196 MPa (2000 kg/cm2) is applied to the inside of the hollow member 11. By means of the applied hydraulic pressure, the axial central portion of the hollow member 11 is expanded radially outwards until it fits tightly against the inner surface of the cavity of the die 6, thereby forming an expanded portion 7 on the axial central portion of the hollow member 11. At this time, in order to prevent the expanded portion 7 becoming thin walled, the hollow member 11 is compressed in the axial direction by the two axial pressing tools 19a and 19b to enhance the supply of material to the expanded portion 7.
There is also a case where the portion expanded in the above manner is further expanded as shown in FIG. 18 (A) and (B). In the aforementioned column bracket 2 formed in a part of the steering column 1 in this manner, it is necessary to form through holes 5 for insertion of a tilt bolt 4 for example. Furthermore, these through holes 5 must be formed after forming the column bracket 2 in which a portion of the metal pipe 3 is plastically deformed. Moreover, in the case of constructing a telescopic steering device, the respective through holes 5 must be formed as long holes in the axial direction of the steering column 1.
As a technique for forming the through holes in the portion of the hollow member which is expanded by the hydroforming method, heretofore, there is known a hydropiercing method or the like as disclosed in Patent Documents 2 and 3, and Non Patent Document 1, all identified below. Three examples of the conventional technique disclosed in the Non Patent Document 1, are described in FIG. 19.
At first, in the case of a first example shown at the left side of FIG. 19, a material provided inside a die 6 is expanded by a hydroforming process to thus complete the step of forming the expanded portion 7. After this, hydraulic pressure is applied to the inside of the expanded portion 7, and in this condition, a punch 9 which is set in a cylinder hole 8 at one portion of the die 6, provided at a position matching with the portion in which a through hole 5 is to be formed, and with a tip end face made with a shape to match with the outer surface of the expanded portion 7, is pressed towards the expanded portion 7. Then, this one portion of the expanded portion 7 is punched out by the punch 9 to thereby make the through hole 5. A blank 10 produced by punching out the one portion of the expanded portion 7 by the punch 9 remains on the inside of the hollow member 11 provided with the expanded portion 7.
Next, in the case of a second example shown in the center of FIG. 19, a material provided inside the die 6 is expanded by a hydroforming process to thus complete the step of forming the expanded portion 7. After this, hydraulic pressure is applied to the inside of the expanded portion 7, and in this condition, a punch 9a which is set in a cylinder hole 8a at one portion of the die 6, provided at a position matching with a portion in which a through hole 5a is to be formed, and with a tip end face inclined in one direction, is pressed towards the expanded portion 7. Then, this one portion of the expanded portion 7 is broken through by the punch 9a, to thereby make the through hole 5a. The shearing or rupture to break through the side wall of the expanded portion 7 in order to form this through hole 5a starts from the one side of the through hole 5a and proceeds gradually towards the other side. Therefore, the blank 10a produced accompanying the working of the through hole 5a remains in a condition connected to the side wall of the expanded portion 7, after completion of the process for the through hole 5a. 
Furthermore, in the case of a third example shown on the right side of FIG. 19, a material provided inside the die 6 is expanded by a hydroforming process to thus complete the step of forming the expanded portion 7. After this, hydraulic pressure is applied to the inside of the expanded portion 7, and in this condition, a slide tool 13 which is set in a punch hole 12 at one portion of the die 6, provided at a position matching with a portion in which a through hole 5 is to be formed, is moved in a direction away from the expanded portion 7. As a result, the tip face of the slide tool 13 and the outer face of the expanded portion 7 which have up to now been in contact, become separated. Since the hydraulic pressure continues to be applied to the inner face of the expanded portion 7, the portion which matches with the punch hole 12 at one portion of the side wall of the expanded portion 7 is strongly pressed to inside the punch hole 12 accompanying the loss of back up, so that it is sheared or ruptured, to thereby form the through hole 5b. In order to scavenge the blank 10b produced as a result, from inside the punch hole 12, this is removed before the next process by for example moving the slide tool 13 forward.
Of these three conventional techniques disclosed in Non Patent Document 1, according to the first example shown on the left side of FIG. 19, the blank 10 which is formed accompanying the formation of the through hole 5 remains on the inside of the hollow member 11. Therefore, after forming the through hole 5, it is necessary to eject the blank 10 from the hollow member 11. However, in the case for example where the end opening of the hollow member 111 is narrower than the size of the blank 10, or the hollow member 11 has a complicated shape, the removal of the blank 10 from inside the hollow member 11 may be impossible or difficult. Moreover, in the case of the first example, accompanying the strong pressing of the outer peripheral face of the expanded portion 7 by the punch 9 in order to form the through hole 5, the portion of the expanded portion 7 surrounding the through hole 5 is deformed (droops) in the radially inward direction of the hollow member 11. Therefore, it is difficult to maintain the shape accuracy and the dimensional accuracy of the surrounding portion after completion of the process.
Next, according to the second example shown in the center of FIG. 19, it is difficult to control the post-processing shape and dimensions of the through hole 5a to a desired accuracy. In particular, the base end portion of the blank 10a is in a connected condition, and hence one end portion (the left end portion of FIG. 19) of the through hole 5a at one portion of the side wall of the expanded portion 7 remains in a condition where the one portion of the side wall is bent and deformed, and the side wall for only the bent and deformed part becomes deformed from drooping. On the other hand, in the central portion or the other end portion (the right end portion of FIG. 19) of the through hole 5a, the side wall is deformed inwards of the expanded portion 7 by the strong pressing in the radially inwards direction by the punch 9a. As a result, in any of the portions, it is difficult to maintain the accuracy in relation to the shape and dimension of the through hole 5a. Furthermore, since the blank 10a remains in the condition where it is projecting in the radial inwards direction from the inner face of the expanded portion 7, then depending on the usage of the hollow member 11, the blank 10a may also become an obstruction.
When these matters are considered, it is preferable to form the through hole 5 in the expanded portion 7 of the hollow member 11 by the third example shown on the right side of FIG. 19. Considering such a situation, the previously considered method such as shown in FIG. 15 for making the steering column 1 provided integrally with the column bracket 2, is described using FIG. 20 to FIG. 23. In this previously considered method, at first, as shown in FIG. 20, the metal tube 3 being the primary material, and in which the plate thickness is T1, is positioned at a predetermined position inside the die 6a. This die 6a, as shown in FIG. 21, is made by bringing together a pair of metal die pieces 15. On the inside, there is provided a hole portion 16 which can fit with substantially no gap, the opposite end portions of the metal tube 3, and the one half portion in the circumferential direction of the central portion, and a cavity portion 17 which protrudes radially outward from the central portion of the hole portion 16. The inner face shape of the cavity portion 17 coincides with the outer face shape of the expanded portion 7 which is to be formed. Furthermore, in one part of each of the pair of metal die pieces 15, is provided punch holes 12a at mutually matching positions, and displaced radially outwards of the hole portion 16 towards the cavity portion 17 from the central axis of the hole portion 16. Furthermore, slide tools 13a are closely fitted inside these punch hole 12a so as to be each moved back and forth with respect to the cavity portion 17.
When making the steering column 1 integrally provided with the column bracket 2, at first, as shown in FIG. 20 and FIG. 21, the metal pipe 3 is fitted inside the hole portion 16 so that the metal pipe 3 is clamped by the pair of metal die pieces 15. In this condition, the other half portion in the circumferential direction of the central portion of the metal pipe 3 faces the concavity 17. Next, while pushing the axial opposite ends of the metal pipe 3 in a direction so that these draw near to each other by means of axial pushing tools 19, hydraulic pressure (typically water pressure) is introduced to inside of the metal pipe 3. This introduction of hydraulic pressure is performed for example through central holes 18 in one or both of the axial pushing tools 19. Furthermore, when performing this operation, in the initial step of the hydraulic pressure introduction, the tip end faces 20 of both slide tools 13a and the inner face of the cavity portion 17 are made to coincide.
When in this manner hydraulic pressure is introduced to inside of the metal pipe 3, and both of the axial pushing tools 19 are moved in a direction to approach each other, the other half portion in the circumferential direction of the axial central portion of the metal pipe 3 is expanded towards the cavity portion 17. That is, a force which compresses the metal pipe 3 in the axial direction is applied while applying a strong force in the radial outward direction on the inner peripheral face of the metal pipe 3, so that the metal pipe 3 is processed into a shape following the inner face shape of the die 6a as shown in FIG. 22 and FIG. 23, that is, into a shape having the expanded portion 7a which protrudes radially outward, on the other half portion in the circumferential direction of the central portion.
If from the condition with the expanded portion 7a formed in this manner, the two slide tools 13a are withdrawn from the side wall 14 of the expanded portion 7a immediately after it has been formed, the portions matching with the two punch holes 12a at one portion of these two side walls 14 are pressed by the hydraulic pressure existing inside of the expanded portion 7a, so that the portions are pressed into inside of these two punch holes 12a, and the through holes 5c are thereby formed in the above portions.
If the technique for forming the expanded portion 7a by the hydroforming method on one portion of the metal pipe 3 in the above manner, is combined with the third example of the conventional technique shown on the right side of FIG. 19, then it is considered to be possible to form the through hole 5c in one part of the expanded portion 7a with good efficiency. However, it has been found from research by the present inventors that if the two techniques are only simply combined, the through hole 5c cannot always be stably formed. The reason for this will be explained by adding FIGS. 24 and 25 to FIGS. 22 and 23.
In the case where the expanded portion 7a is formed on one portion of the metal pipe 3 by means of hydroforming, then at the part corresponding to the expanded portion 7a, the metal plate constituting the metal pipe 3 is stretched in the surface direction. Therefore, the metal pipe 3 is compressed in the axial direction, to urge the supply of material to the expanded portion 7a, however the metal plate still becomes less than the original plate thickness T1 (refer to FIG. 20). Moreover, the degree that the plate thickness becomes thinner in this manner varies within the expanded portion 7a. More specifically, since the supply rate of the material is reduced with distance from the base (the bottom portion in FIGS. 22 and 23) of the expanded portion 7a, the extent to which the plate thickness becomes thin at the portion towards the base is reduced, and the extent to which the plate thickness becomes thin towards the tip end portion (the top portion in FIGS. 22 and 23) is considerable. Furthermore, the curvature of the tip end portion also becomes great (the radius of curvature becomes small), so that at the left and right two corner portions of FIG. 23 and in the vicinity thereof, the amount that the plate thickness is reduced is remarkable.
Moreover, the plate thickness of the two side walls 14, at the portion where the two through holes 5c are to be formed becomes a non-uniform condition (gradually changing) in relation to the width direction (the vertical direction in FIG. 22 to FIG. 25) of these two through holes 5c. More specifically, the cross-section shape of the two side walls 14, at the portion where the through holes 5c are to be formed becomes a wedge shape. Moreover, the plate thicknesses T2 and T3 (refer to FIG. 24) of the two side walls 14 at the two edge portions in the width direction of these two through holes 5c become thick towards the base end of the expanded portion 7a, and similarly become thin towards the tip end (T2>T3).
When forming the expanded portion 7a, the pattern for the pressure rise of the hydraulic pressure introduced to inside the metal tube 3, and the pattern (axial pressing pattern) for advancing the axial pushing tools 19 are appropriately set. That is, in the case where the increase in hydraulic pressure with respect to the increase in the axial pressing amount is fast, reduced thickness of the expanded portion becomes pronounced, so that there is a high possibility of cracking occurring. On the other hand, in the case where the increase in the pressing amount is given precedence over the increase in hydraulic pressure, then buckling of the material is likely to occur. In general, axial pressing is given precedence within a range where buckling does not occur, and if the final axial pressing amount is set large, the difference between the thicknesses T2 and T3 at the two end portions, and the difference from the original thickness T1 can be made small. For the inside-out hydropiercing as shown on the right side of FIG. 19, in the case where the normal working method as disclosed in Non Patent Document 1 is adopted, then of the respective plate thicknesses T1, T2 and T3, the difference for the plate thicknesses T2 and T3 of the two end portions, when viewed from the side where the plate thickness is thick, preferably is within 5%, and more preferably is within 3%, from the aspect of forming the two through holes 5c. However, in the case where asymmetry of the product shape is remarkable, then even if the axial pressing pattern or the hydraulic pressure increase pattern is adjusted, the non-uniformity of the plate thickness cannot be sufficiently cancelled. In particular, in the case where as shown in FIG. 20 to FIG. 23, the expanded portion 7a is formed on only one side of the metal pipe 3, then as mentioned above, the plate thickness of the side wall 14 where the through hole 5c is to be formed becomes non-uniform. In other words, the situation also arises where the difference of the plate thicknesses T2 and T3 of the two end portions, when viewed from the side where the plate thickness is thick, exceeds 3%, or even exceeds 5%.
In this manner, even though the thickness of the side wall 14 where the through hole 5c is to be formed may become non-uniform, as shown in FIG. 21, and FIG. 23 to FIG. 25, in the case where the slide tool 13a with the tip end face 20 being a smooth face parallel with the two side walls 14 is used, it is difficult to stably form the through hole 5c. That is, in the case where the slide tool 13a with the tip end face 20 thereof in the aforementioned simple shape is used, then if the shape of the through hole is complicated such as an oval shape or an ellipse shape etc., or even with a simple circular hole, in the case of forming a through hole where the aperture area is large, the blank cannot be completely removed from the portion which becomes the through hole, so that this blank is likely to remain in a condition partially connected to the material. In particular, as with the case where the through hole 5c is formed in the side wall 14, in the case where a through hole is formed in a portion where the plate thickness is uneven, the problems as mentioned above are likely to occur.
That is, even though there may be a difference of more than 5%, in the plate thicknesses T2 and T3 of the opposite end portions, in the case where the through hole 5c is formed as mentioned above using the slide tool 13a with the flat end face 20, then at approximately the same time as when the slide tool 13 starts to move back, the portion facing the punch hole 12a at one part of the side wall 14 starts to be deformed (strained) towards the inside of the punch hole 12a. Then, at the point in time when the slide tool 13a has moved back a certain amount, the portion facing the punch hole 12a at the one part of the side wall 14 where the plate thickness T3 is thin, becomes fractured prior to the portion facing the punch hole 12a at the one part of the side wall where the plate thickness T2 is thick. As a result, as shown in FIG. 25, the portion facing the punch hole 12a at the one part of the side wall 14, where the plate thickness T2 is thick, remains connected to the side wall 14, under a condition where the same hydraulic pressure exists on both sides of the portion which is to be punched out. That is, the hydraulic pressure on the inside of the metal pipe 3 is released from the rupture location. As a result, even if the slide tool 13a is moved back more than this, the fracture of the portion connected to the side wall 14 where the plate thickness T2 is thick does not progress, so that the through hole 5c can no longer be formed. In this manner, a phenomena where one part of the portion to be punched out remains in a condition connected to the side wall 14, becomes more remarkable the greater the difference between the metal thickness of the portions where the through hole is to be formed, or the more difficult the shape such as the oval shape, than for when the shape of the through hole is a round hole.
As shown in FIG. 19, in the case of the heretofore known hydropiercing, the expanded portion 7 becomes a symmetrical shape (or approximately symmetrical shape) in relation to the central axis of the hollow member 11, and the metal thickness of the tube wall of the portion where the through hole is to be formed is substantially uniform. Therefore, even in the case of so called inside-out hydropiercing where the portion to be punched out is punched out to the radial outside, the through hole can be formed. However, in the case of the column bracket 2 of the steering column 1, making the plate thickness of the tube wall of the portion where the through hole is to be formed uniform is difficult, as mentioned before. Furthermore, in the case of the hole forming method described for the left and the center of FIG. 19, even if the plate thickness of the portion where the through hole is to be formed is uneven, the shape itself of the through hole is possible, however there are the aforementioned problems.
In the method described in Patent Documents 2 and 3, the overall process is complicated, so that an increase in cost cannot be avoided. Consequently, the situation as shown in FIG. 20 to FIG. 23 where the working for the expanded portion 7a and the operation for forming the through hole 5c are made successive to improve the efficiency, is not an alternative method for reducing cost.
Patent Document 1: Japanese Patent Application Publication No. H8-276852
Patent Document 2: Japanese Patent Application Publication No. H6-292929
Patent Document 3: Japanese Patent Application Publication No. 2001-314926
Non patent document 1: Frank-Ulrich Leitloff and Steffen Geisweid, “Application of Tube Hydroforming Technology to the Production of Automotive Components”, Journal of the Japan Society for Technology of Plasticity Vol. 39 no. 453 (1998-10)